1. Field of the Invention
The present invention relates to interferrometric fiber optic gyros (IFOG), and more particularly to lowering the total gyro noise in an IFOG.
2. Description of the Related Art
There is a growing demand for high accuracy gyros for satellite pointing applications. In order to improve gyro sensitivity, it is necessary to lower the total gyro noise. In typical satellite pointing applications, an interferrometric fiber optic gyro (IFOG) is employed. In the IFOG, noise elements arise from both the optical and electrical elements.
A closed loop IFOG is illustrated in FIG. 1. An IFOG generally includes a light source 10, a coupler 20, an integrated optics chip 30, and a fiber coil 40, which comprise the optical circuit 5. The fiber coil 40 provides the rotation-sensitive interferometer. The processing electronics 45 of the IFOG generally comprise a photodetector 50, an amplifier/filter 60, an analog-to-digital converter (A/D) 70, a digital signal processor (DSP) 80, a digital to analog converter (D/A) 90 and amplifier 95.
The processing electronics 45 function to provide a feedback phase shift in the optical circuit 5 which effectively nulls out a rotation-induced phase shift sensed in the fiber coil 40. The magnitude of the feedback phase shift corresponds to the rotation rate. The photodetector 50 converts an optical power output from optical coupler 20 to a corresponding voltage. The corresponding voltage is processed by amplifier/filter 60 and translated to a digital signal by A/D converter 70. The corresponding feedback signal is calculated in DSP 80, and fed back into the gyro with a modulating square wave via D/A converter 90 and amplifier 95.
In a square wave modulated IFOG, a rotation about a rate input axis 41 of the fiber coil 40 produces input signals at the photodetector 50, which are denoted as xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d in FIG. 1A. A difference Axe2x88x92B corresponds to the sensed input rotation rate and is designated Derror. The input signal characteristics result from interference patterns of the counter propagating light waves that travel in the fiber optic coil 40. The sharp spikes present in the waveform are a result of the modulated signals driving the interference patterns through the peak of the interference curves.
An accurate measurement of the magnitude of the A and B levels of the photodetected signals is crucial to the overall performance of the IFOG. However, this measurement accuracy of the low level xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d signals is compromised by the presence of noise and the high level signal generated by the optical spikes. A high gain is required to maintain the required closed loop performance, but results in signal distortion due to saturation effects from the high optical spike signals in the signal processing electronics 45, most notably in the photodetector 50 and amplifier/filter 60.
Prior art signal processing methods utilize very fast D/A converters 90 and amps 95 in modulation schemes to minimize the width of the optical spikes. The photodetector 50 is typically comprised of a photodiode 52 and a wide bandwidth transimpedance amplifier 54 to maintain signal fidelity. The amplifier/filter 60 is typically comprised of several stages of high gain amplification with clipping circuitry and filters to extract the desired A and B signals. The wide bandwidth of the photodetector 50 and amplifier/filter 60 make them susceptible to noise pickup.
Additionally, digital techniques using multiple samples or oversampling are typically employed to average the noise in each A and B sample. The use of oversampling increases system speed requirements and requires the use of high-speed logic and a fast DSP 80. The fast logic and higher clock speeds increase the likelihood of noise coupling into the high gain wide band photodetector 50 and amplifier/filter 60, which ultimately degrades IFOG performance.
Therefore, an improved apparatus and method for signal conditioning to reduce IFOG noise are needed.
It is therefore an object of the present invention to provide an apparatus for reducing an overall IFOG noise.
It is another object of the present invention to provide an improved method of signal conditioning in an IFOG.
To achieve the above objects, a signal conditioning circuit to reduce an interferrometric fiber optic gyro (IFOG) noise is provided in accordance with the present invention, the IFOG including a light source, IOC and coupler to output an optical power signal corresponding to a rotation-induced phase shift in a fiber coil of the IFOG. The signal conditioning circuit comprises a photodiode that converts the optical power signal to a corresponding electrical compensation signal and at least one digital controlled (switched) integrator that processes the electrical compensation signal. Selectively switching the integrator or integrators between a plurality of modes effectively produces a compensation signal with lower noise characteristics.
A method of signal conditioning in an IFOG to reduce IFOG noise is also provided in accordance with the present invention. The method comprises the steps of converting the optical power signal to a corresponding electrical compensation signal by selectively integrating a current of the electrical compensation signal using at least one switched integrator.